The present invention relates to a method of severing a body of vitreous or vitrocrystalline material by irradiating the body with at least one laser beam. The invention also relates to an apparatus for the performance of the method.
It is known to sever glass by means of a laser beam. The use of a laser beam has been the subject of research by prominent glass manufacturers. The teaching resulting from this research is that the laser beam should have a wavelength such that the energy absorption by the glass is as high as possible. Such teaching is provided, for example, by United Kingdom Patent Specification No. 1,246,481. More specifically, the prior teaching proposes the use of a CO.sub.2 --CO.sub.2 --N.sub.2 --He laser producing a radiation wavelength of 10.6 microns.
In the case of thick vitreous sheet material, it has been found that when attempting to use a CO.sub.2 --N.sub.2 --He laser, the material is often not severed or is not cleanly severed along the intended line, the severed edges exhibiting irregularities or jagged portions. The failure to reliably and consistently achieve a clean severance is particularly notable in the case of vitreous sheet material having a thickness in excess of 5 mm.
One of the factors which influences the results when irradiating the vitreous sheet material by means of a laser beam is the amount of radiation which is incident upon each point of the material along the intended line of severance. In order to cut relatively thick sheets, one would accordingly expect that the poor results above referred to were attributable to an insufficient energy absorption concentration at each irradiated point. Attempts have therefore been made to improve the results by increasing the energy of the laser beam.
It has been found that the required results cannot be achieved in this way. A clean severance of the vitreous sheet material does not take place. With an increase in the energy of the radiation incident upon a sheet of given thickness above 5 mm there is increasing tendency for the severance, when it occurs, to create chipped or jagged fragile edges. Moreover, in the case of very thick sheets the energy of the incident radiation has to be increased to such an extent that the first noticeable effect of the irradiation is a softening of the surface layer of glass, which of course precludes all possibility of the clean severance which it is desired to achieve.
Quite apart from the fact that the use of higher radiation dosages does not give the desired results, it is undesirable because it involves either more power consumption or a slower relative speed between the laser beam and the sheet material. In the flat glass production industry the need is for rapid cutting of the glass. This is notably so in the cutting of a ribbon of drawn sheet glass as it leaves the drawing machine. In the drawing of sheet glass by the Pittsburgh process, for example, the drawing speed may be of the order of 30 m per hour for glass 6 mm in thickness and the speed at which the ribbon continuously cut to remove its thick margins has to keep pace with this drawing speed.